Method and system for wellbore protection when explosively stimulating earth formations

ABSTRACT

A METHOD FOR EXPLOSIVELY FRACTURING AN EARTH FORMATION INCLUDES THE PLACEMENT OF A SHOCK ABSORBENT LINER IN THE WELLBORE ADJACENT THE FORMATION TO PREVENT WELLBORE DAMAGE WHEN AN EXPLOSIVE IS DETONATED IN THE WELLBORE FORMATION.

METHOD ANI) FIG.

INVENTOR WILLIAM L. HILL ATTORNEY FIG. 2

United States Patent O U.S. Cl. 1664-299 Claims ABSTRACT OF THE DISCLOSURE A method -for explosively fracturing an earth formation includes the placement of a shock absorbent liner in the wellbore adjacent the formation to prevent wellbore damage when an explosive is detonated in the wellbore and formation.

BACKGROUND oF THE INVENTION This invention relates to an improvement in explosively fracturing formations. Extraction of oil or gas from for- -mations penetrated by wellbores often times is complicated Vby the lack of"`permeability in the formation. In order to maximize production from such low permeability formations, it is often necessary to fracture the formation, and thereby increase permeability. There are two basic methods of creating?v fractures in a formation. One is to create hydraulic fractures by applying a pressurized uid. against the formation until the formation splits. Another method is to detonateY explosives in the lformation to create a shock wave which fractures the rock matrix of the formation.

The use of explosives as a well stimulation tool dates back to the-1860s. The lirst wells were shot with nitroglycerin, and this method of stimulation was the standard mode until hydraulic fracturing was introduced in 1949. Recently, new interest in stimulating Wells with explosives has been genet-ated `by the development of improved explosives and new methods of using them.

There are two basic methods of explosive fracturing. One is to detonate the explosive in the wellbore, and the other is to detonate the explosive in the formation adjacent to the wellbore. When the explosive is confined in the wellbore, the result of .detonation is a cylindrical rubble zone in the vicinity of the wellbore surrounded-by a system of vertical fractures radiating like wheel spokes from the rubble zone. This result is caused by the nature of a detonated explosive. Upon detonaton, the explosive vundergoes a very rapid self propagating exothermic decomposition. This decomposition yields more stable products inthe form of gases which exert tremendous pressure as they expand at the high temperature generated by the release of heat. 'Ilhis rapid release of energy creates a shock wave. The rock matrix adjacent to an explosive charge will be shattered as the shock wave moves into the rock matrix. The shock wave in the rock consists of two components, a compression wave and a shear wave. When the energy level of either of these wave exceeds the strength of the rock under dynamic `loading, the rock will fail, thus creating a fracture network. The gases generated in the explosion obtain a pressure on the order of one million pounds per square inch which pushes against the exposed surfaces of the fractured rock matrix. The expansion of the gases will extend the fractures until its energy for doing work is dissipated.

When explosives are placed in the formation, they are placed in a fracture which is usually created by hydraulic means. Such hydraulic fracturing may result in horizontal or vertical fractures. When an explosive is located in a horizontal fracture and detonated, the high pressure shoe-k ICG wave would shatter the adjacent surfaces of the fracture. As the shock wave moves upward and downward from the plane of detonaton, it will traverse various strata. As the wave moves through a density discontinuity, part of the Wave Will be reflected back as a tension wave. The tensile strength of rock is several orders of magnitude less than the compressive strength, therefore, new fractures will be created by a tension wave. A rubble zone will be created in the fracture, and the flow conductivity of the fracture will depend on the thickness of the rubble zone and the grain size of the rubble.

Since t-he net thickness of the explosive layer is small, fractures generated in the rock matrix beyond the rubble zone will not be extensive. This is because the volume of gaseous products available for extending the fractures is directly related to the amount of explosive present. Since the explosive is present as a thin layer, a limited quantity of gas is available per unit surface area of the fracture. The extension of the original fracture radially is limited because the large volumetric increase required rapidly dissipates the energy in the available gas volume.

Detonations occurring in vertical fractures yield similar results as those occurring in horizontal fractures. Lateral extension of the original fracture occurs more readily, since the vertical height of the fracture is confined by the stratigraphic boundaries, thus requiring a smaller volumetric increase for fracture extension.

Several methods have been used to detonate explosives in the formation. One method is to place each component of a multiple component explosive system into the formation separately. Upon each component contacting and mixing with one another, a hypergolic mixture is formed. Various problems relate to this technique. A few of the reasons why suc-h a system is not successful are: detonations in areas other than the formation, poor eliiciency caused by non-homogeneous mixing of the components, and a lack of 'propagation of the explosion due to such non-homogeneous mixing of the components.

The most reliable system for detonating an explosive in the formation is to place sufficient explosive in vthe formation so that a portion remains in the wellbore, and then detonating the explosive in the wellbore by usel of a time bomb. The main problem with this type of detonation is that of wellbore damage caused by the quantity of explosive located in the wellbore and the creation of a rubble zone adjacent the wellbore with resultant Sloughing of the formation.

It is an object of the present i-nvention to provide an improved explosive stimulation system.

SUMMARY ORIT-IE INVENTION With this and otli'er objects in view, the present invention contemplates; a system for explosively stimulating earth formations. A shock absorbent liner is located in the wellbore adjacent the formation and perforated. The

a formation is hydraulically fractured and an explosive is located in the fracture and wellbore and detonated.

BRIEF DESCRIPTION OF THE |DRAWINGS FIG. 1 is a cross section of a fractured formation penetrated by a wellbore having a liner and a cylinder in the wellbore; and

|FIG. 2 is a cross section of the formation shown in FIG. l where the liner is coated on the interior surface of well pipe.

DESCRIPTION OIF 'IH-E PREFERRED EMBODIMENTS FIG. l illustrates a wellbore 12 which extends through a formation 22 and terminates with a rathole 34. The formation 22 is defined by an upper boundary 24 and a lower boundary 26. This upper boundary 24 and lower boundary 26 may define the boundary of adjacent impermeable formations, fault zones, a water or gas zone interface, or merely a change in formation permeability. Rathole 34 is the portion of the wellbore located below the formation 22. Positioned in the rathole 34 is pea gravel 32 which is l" to 1A" in diameter. The pea gravel 32 occupies all but the upper portion of the rathole 34. Lying on top of the pea gravel 32 and sealing the rathole 34 from the rest of the wellbore 12 is an impermeable material 36 such as neat cement or plaster of parls.

Formation 22 is shown as having a yvertical fracture 20 extending from each side of the wellbore 12. This fracture would normally have been created by standard hydraulic fracturing techniques, and is shown as being propped open by propping agent 44. The propping agent 44 may be sand, gravel, nut shells, fruit pits, glass, 0r similar material. Wellbore 12 contains well pipe 14 which is bonded to wellbore 12 by cement 28. The well pipe 14 extends from the surface to a point adjacent the top of formation 22. Attached to and hanging below well pipe 14 is liner 16, which is a shock absorbent pipe member constructed of materials such as glass fiber or plastic material. The liner 16 should be made of a material which does not readily transmit shock waves. This liner 16 is located adjacent to the formation 22, and is also bonded to the wellbore 12 by cement 28. Perforations 18 extend through the liner 16 and adjacent cement 28 so that the interior of wellbore 12 communicates with formation 22.

In the interior of wellbore 12 adjacent the liner 16 and formation 22 is enclosed cylinder 30. Cylinder 30 is preferably made of a shock absorbent material. The cylinder 30 may be made of many varied materials because its main functions are simply to occupy space in the wellbore and absorb shock. The cylinder may be a hollow plastic filled with an aggregate or it may be a weighted cellular plastic material. lPositioned atop cylinder 30 is time bomb 38 having timing mechanism 40. Located in the vertical fracture 20 and in the wellbore adjacent the formation 22 is an explosive luid. The upper surface of this explosive fluid is shown at 42 and partially immerses time bomb 38.

'Ihe use of the pea gravel 32 with the related impermeable material 36 and the cylinder 30 are disclosed in a copending patent application filed of even date herewith, entitled Method of Explosively Stimulating Earth Formations.

The system just described is basically for the purpose of precluding or diminishing wellbore damage or creation of a rubble zone adjacent the wellbore. The well pipe 14 when run into the wellbore 12, has a liner -16 attached to its lower end, which liner is located adjacent formation 22. .After the well pipe and attached liner 16 are positioned, they are cemented into place by placing cement between the well pipe and liner and the wellbore 12. Once this has been completed, the liner and adjacent cement is perforated to allow communication with the formation 22. At this point, the formation is hydraulically fractured by applying a fracturing fluid containing a propping agent 44 against the formation 22 at a pressure sutlicient to part the formation. Once the fracture has been created, the pressure is no longer applied against the formation, and the fracture 20 is maintained open by propping agent 44.

After the fracturing step has been accomplished, the pea gravel 32 is placed in the rathole 34 by conventional means such as a bailer. A space is left between the pea gravel and the bottom of the formation 22 to leave room for the impermeable material 36. This impermeable material 36 may also be placed by a bailer. After the impermeable material 36 sets up, the rathole 34 is elfectively separated from the interior of wellbore 12.

Next, the cylinder 30 is lowered by wireline into the wellbore 12 and is positioned so as to rest on top of the impermeable material 36. The cylinder 30 is sized such that there is an annular space between the cylinder 30 and the liner 16. Following positioning of the cylinder 30, an explosive lluid is placed in the fracture 20 and in the wellbore adjacent the formation 22. The placement of the explosive fluid is also accomplished by use of a bailer, in the same manner as the placement of the pea gravel 32 and impermeable material 36. A quantity of explosive fluid is used which is sufficient to iill the wellbore to a point slightly above cylinder 30. The timing mechanism 40 of time bomb 38 is then set at the surface such that sufficient time is available for locating the time bomb 38 in the wellbore 12. After the time bomb 38 has been set, it is lowered into the wellbore 12 and allowed to come to rest atop cylinder 30. When the time bomb 38 cornes to rest, it will be partially immersed in the explosive fluid. When the timing mechanism 40 activates the time bomb 38, the explosive fluid is detonated.

In order for the explosion to propagate in the wellbore 12, the cylinder 30 must be sized such that the annular space between the cylinder 30 and the liner 16 is in excess of the critical propagation diameter of the explosive. Testing by the U.S. Bureau of Mines has shown that nitroglycerin will propagate through a thickness of 132" for a distance of 12 feet. To insure that'the cylinder 30 does not become inclined and reduce the annular space in localized areas, conventional wellbore centralizers may be attached to the cylinder 30.

Once the explosion propagates through the wellbore 12, it enters the formation 22 through perforations 18, whereupon the explosive fluid in the fracture 20 is also detonated. Such detonation in the fracture serves to shatter the rockmatrix adjacent the fracture plane 20. Another result of the detonation is that the original fracture 20 is laterally extended.

Use of the liner 16 serves to dissipate the amount of shock transmitted to the formation 20 adjacent the wellbore 12. The cylinder 30 reduces the amount of explosive fluid contained in the wellbore so as to provide a reduced amount of shock being transmitted to the liner 16. The reduced amount of shock transmitted to the formation results in a reduction or elimination of a rubble zone adjacent the wellbore. Such a rubble zone, if formed, may results in sloughing of the formation 22 such that impermeable material located in adjacent formations may fall in the area adjacent the perforations and thereby reduce substantially the permeability necessary for production.

FIG. 2 illustrates a variation of the use of the liner 16 shown in FIG. 1. Again there is illustrated a formation 22 penetrated by a wellbore 12 having well pipe 14 cemented to the wellbore 12 by cement 28. The well pipe 14 in this instance extends through the formation 22. Coated on the interior of the joint of well pipe 14 positioned adjacent the formation 22 is liner 16. This liner is made of the same type material as that described in FIG. l. Perforations 18 extend through the liner, well pipe, and cement so that the interior of the wellbore 12 communicates with the formation 22. Formation 22 has upper and lower boundaries 24 and 26 and a vertical fracture 20 having propping agents 44 therein maintaining separation of the walls of the fracture 20. Time bomb 38 is shown at the bottom of the wellbore 12 having timing mechanism 40 which is a clock mechanism for activating the time bomb 38 by electrical, chemical, or other energizing means.

In the operation of the process for explosively stimulating the formation 22 with the system shown in the FIG. 2, it is necessary to coat the interior wall of a joint of well pipe with a shock absorbent liner prior to running the joint into the wellbore adjacent the formation 22. The well pipe is cemented into position and the specially coated joint of well pipe is then perforated. Next, the formation 22 is fractured through Perforations 18 and propped open by propping agent 44 whichwas carried into the formation 22 by the hydraulic fracturing fluid.

An explosive fluid is then placed in the formation fracture 20, and the wellbore adjacent to the formation 22.

The clock mechanism of the time bomb is set at the surface to provide sufficient time for placing the time bomb in the wellbore. The time bomb 38 is then placed in the bottom of the wellbore 12 and when the timing mechanism 40 activates the time bomb 38, the explosive in the wellbore and the adjacent fracturing zone is detonated.' The shock wave created by the explosive in the wellbore is at least partially absorbed by the liner 16 so that extensive wellbore damage does not occur. Accordingly, the rubble zone adjacent the wellbore 12 would be minimized. It is contemplated that a substance similar to the cylinder 30 may be placed in the wellbore adjacent the formation in order to further minimize the explosive shock wave. Additionally, an appreciable rathole below the formation is present, it should be sealed off in a manner such as that shown in FIG. 1.

Once the explosive has been detonated, any debris left in the wellbore is then removed. The impermeable layer 36 and pea gravel 32 shown in FIG. 1 together with cylinder 30 would all be fragmented and easily removed by well -known wellbore clean-out methods.

While particular embodiments of the present invention have been shown and described, it is apparent that changes' and modifications may be made without departingfrom this invention in its broader aspects, and therefore, the aim in the appended claims is to cover all such changes and modifications which fall within the true spirit and scope of this invention.

What is claimed is:

1. The process of explosively stimulating an earth formation penetrated by a wellbore having well pipe therein comprising: attaching a cylindrical non-metallic shock absorbent liner to the well pipe, which liner extends through the formation to be stimulated; perforating the liner so that the interior of the liner and the formation are in communication; injecting an explosive uid into the formation fracture and the adjacent wellbore; and detonating'; the explosive uid.

2. The process of claim 1 including, prior to perforating, the steps of attaching thuerliner to the interior of well pipe, and cementing the well pipe with attached liner to Ithe wellbore. j

3. The process of claim 1 including, prior to perforating,- the steps of attaching the liner to well pipe so that the. liner hangs below the well pipe, and cementing the well pipe and liner to the wellbore.

4. The processhof claim 1 including, prior to injection of the explosive fluid, the step of locating a substance in the wellbore sized so that the substance occupies the -major portion of the wellbore adjacent the formation being fractured and wherein the substance is arranged to provide a void space in the wellbore adjacent the formation, larger than the critical explosive propagation diam eter of the explosive fluid.

5. The process of claim 1 including the step of hydraulically fracturing the formation prior to injecting and detonating the explosive uid.

6. The process-(of claim 1 wherein the liner is constructed of a friable material.

7. A system for explosively stimulating an earth formation which has -been hydraulically fractured through a wellbor'le comprising: well pipe cemented to the wellbore; a friable shock absorbent liner located adjacent the formation being explosively stimulated, said Iline being attached to the well pipe; and an explosive fluid located in the formation fracture.

8. The system claim 7 wherein said shock absorbent liner is threadedlyattached to and hangs below the well pipe and is cemented to the wellbore 9. The system in claim 7 wherein said liner is a coat ing of plastic material on the interior surface of the well pipe located adjacent the formation 10. The system of claim 7 including a substance, located in the wellbore adjacent the formation, sized to occupy the major portion of the wellbore adjacent the formation and arranged to provide void space larger than the critical explosive propagation diameter of the explosive lluid.

References Cited UNITED STATES PATENTS 3,075,463 1/1963 Eilers et al.. 166-299 3,191,678 6/19 65 Hinson 16e-299 2,892,405 6/1959 Chesnut 16s-299 40 2,851,112 9/1958 Buck 16s-242x 3,055,424 9/1962 Allen 16s- 242 x 2,911,046 11/1'959 Yahn 16s-299 x 3,235,007 2/1966 Kem et a1 16e-280 r STEPHEN I. NOVOSAD, Primary Examiner U.S. Cl. X.R. 166-308 

